When Virginia’s mag­ni­tude 5.8 earth­quake hit last August, the first Twitter reports sent from people at the epi­center began almost instantly at 1:51 p.m. €”and reached New York about 40 sec­onds ahead of the quake’s first shock waves, according to cal­cu­la­tions by the social media com­pany SocialFlow. The flood of mes­sages peaked at 5,500 tweets a second.

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